Unijunction transistors



May 24, 1966 T. P. SYLVA N UNIJUNCTION TRANS ISTORS Filed March 2-3, 1962 EMITTER VE you-s FIG.3.

' PEAK POINT I5 NEGATIVE RESISTANCE REGION 6-- VBB 20V SATURATION 2- VALLEY POINT /6 REGION I? l l l I o 40 50 I EMITTER CURRENT (MILLIAMPS) FIGZ,

INVENTOR: TAGE P. SYLVAN Hl A TORNEY.

United States Patent 3,253,196 UNIJUNCTION TRANSISTORS Tage P. Sylvan, Liverpool, N.Y., assignor to General Electric Company, a corporation of New York Filed Mar. 23, 1962, Ser. No. 182,057 4 Claims. (Cl. 317234) The present invention relates to semiconductor devices and, in particular, to improvements in semiconductor devices of the type commonly referred to as unijunction transistors or double base diodes. Such devices are disclosed and claimed in Patent 2,769,926-Lesk, and Patent 2,907,934Enge1, both assigned to the assignee of the present invention.

The unijunction transistor disclosed in these patents comprises a bar of semiconductor material having a noninjecting electrode or base located at one end of the bar and making ohmic contact therewith, another non-injecting electrode or base at the other end of the bar and making ohmic contact therewith, and a minority carrier injecting electrode or emitter located intermediate the ends of the bar. In the operation of such a device, a uni directional voltage is applied between the electrodes to produce a potential in the bar in the vicinity of the emitter. A voltage is also applied between one of the non-injecting electrodes (to be referred to as base one) and the emitter in a direction tending to forward bias the emitter with respect to the bar. When the latter voltage is below a certain critical value, referred to as the peak point voltage, the emitter is reversely biased and is nonconducting. As the critical value of the voltage is reached and exceeded, the emitter becomes forward-biased and conducts, thereby causing injection of minority carriers into the bar and lowering of the conductivity of the region between the emitter and. base one until the region is saturated with conduction carriers.

A graph of voltage versus current between the emitter and the base one appears somewhat as follows. For voltages at the emitter less than the voltage at which it becomes forwardly biased, current increases with voltage but is quite low. As voltage is raised, the peak point voltage is reached and the current increases rapidly. With increase in current, voltage drops very rapidly to a minimal value called the valley point and thereafter increases slightly with increase in current. Of course, the current now conducted between the non-injecting electrodes increases because of the drop in voltage in the bar. The ratio of the voltage between the peak point and the valley point to the voltage applied between the base electrodes of the unijunction transistor is referred to as the stand-off ratio and is designated by the symbol 1 The unijunction transistor devices described are useful in a variety of circuit applications, such as sawtooth oscillators, as counters, as switches, as triggering devices for such devices as silicon controlled rectifiers. In these applications the parameters of the devices which are important are stand-off ratio, the time required to switch the device from a non-conductive condition to a conductive condition, the time it takes the device to recover to its initial state after removal of voltage on the emitter and the saturation voltage. Of secondary importance are parameters such as emitter leakage current, peak point current and valley point current Limits in such parameters as stand-off ratio, turn-on time and emitter saturation voltage have been reached in conventional designs.

The present invention is directed to providing substantial improvements in such parameters as the above in unijunction transistors than is possible in conventional unijunction transistors, and at the same time providing a device which is simpler and substantially less costly to manufacture.

3,253,196 Patented May 24, 1966 Accordingly, it is an object of the present invention to provide a unijunction transistor having a higher stand-off ratio than hitherto obtainable in conventional unijunction devices.

It is also an object of the present invention to provide a device with much lower emitter saturation voltage along with higher stand-off ratio than in conventional unijunctional transistor devices.

It is also an object of the present invention to provide a unijunction transistor which can be switched from a non-conductive to a conductive state and back again in a much shorter time than in conventional devices of such character.

It is also an object of the present invention to provide a device requiring much smaller emitter triggering currents and having much lower emitter leakage currents.

It is also an object of the present invention to provide a unijunction transistor which can be made by a variety of processes.

It is also an object of the present invention to provide a semiconductor device which can be made from a minimum of elements which can be very easily fabricated, yet which has superior electrical characteristics.

It is a further object of the present invention to provide a unijunction transistor of high reliability.

The present invention is carried out in one illustrative form thereof in a semiconductor device comprising a body of semiconductor material of one conductivity type by the provision of a small area non-injecting or ohmic contact to one portion of the body and a large area noninjecting or ohmic contact to another portion of the body remote from said one pon'on whereby equal resistance planes in said body separated from one another by equal resistance values are radially spaced farther apart remote from the small area contact than close to the small area contact. An injecting contact or emitter is also provided in a region adjacent the small area contact. With such an arrangement, greater stand-01f ratios can be achieved for the device with a much closer spacing between the base one electrode and the emitter electrode than in devices having conventional bar-type geometries with the result that the many advantages enumerated above are obtainable.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in accordance with the accompanying drawings, in which:

FIGURE 1 shows a perspective view of a semiconductor device showing an embodiment in accordance with the present invention;

FIGURE 2 shows a sectional view of the embodiment of FIGURE 1;

FIGURE 3 shows a graph of the emitter characteristics of the device of FIGURE 1;

FIGURE 4 shows a sectional view of another embodiment of the present invention;

FIGURE 5 shows a side View of a further embodiment in accordance with the present invention; and

FIGURE 6 represents a top view of the device of FIG- URE 5.

Referring now to FIGURES 1 and 2, there is shown a device comprising a header member 1 and a cap member 2. The header member 1 comprises an insulating base portion 3 conveniently made of glass in which are embedded a group of leads 4, 5 and 6. Leads 4 and 5 extend through openings in the platform portion 7 of conductive member 8 surrounding the glass 3. Lead 6 is welded to the conductive member 8. The conductive member 8 is preferably made of some alloy such as fernico, containing by weight 50% iron, 33% nickel and 17% cobalt, which has a coefficient of expansion similar to the coefficient of expansion of glass. The platform 7 is conveniently gold-plated. A wafer or planar pellet 10 of N-type conductivity and a resistivity of 120 ohm-centimeters is soldered to the platform 7 by means of a goldantimony (by weight 99% gold, 1% antimony) solder which can be in the form of a preform placed between the platform and the planar pellet and melted to cause it to fuse to the platform and make ohmic contact therewith. A small area non-injecting contact is made to the upper face of the wafer by means of gold-antimony wire 11 (by weight 99% gold, 1% antimony) by fusing the wire thereto. The other end of the wire is welded to the lead 4. An injecting contact is also provided in the upper surface of the planar pellet 10 by means of an aluminum wire 12 which is fused to the surface of the pellet. The other end of the wire is welded to the lead 5. After such fabrication of the device, of course, cap 2 is secured in place by mating and welding flange portions of members 1 and 2.

In the embodiment shown, the aluminum wire is conveniently three mils (three-th-ousandths of an inch) in diameter and is fused to the wafer by passing a pulse of current through the wire and the planar pellet (for example, /2 ampere for one second). The gold-antimony wire is conveniently two mils in diameter and similarly is secured to the upper surface by passing a pulse of current through the wire and the planar pellet (for example, one ampere for one second). It should be noted that the dimensions of the pellet can be changed without altering significantly any of the characteristics of the resultant device as long as the requirements are met that non-injecting contact area is small with respect to the other dimensions of the semiconductor wafer 10 including the other base contact. The emitter contact may be comparable insize to the small area base one contact.

With a small area base one contact, most of the voltage drop between base one and base two occurs in the vicinity of the small area contact because of spreading resistance effects. Consequently the emitter contact can be placed much closer to the base one contact for a given stand-off ratio with the result that switch-on time is considerably reduced as well as the other parameters of the device, as is readily apparent from Table I below which sets forth the parameters for the device of FIGURE 1 in comparison to a conventional device using bar-type geometry design.

The device described is made of materials which are inexpensive, which can be put together with a minimum number of operations, and do not require any particular critical operations. Having determined the size of the wafer and the size of the wires and the stand-off ratio de- .sired, the most critical consideration is the location of the 'wire forming the small area base one contact with respect to the wire forming the emitter contact. Such location can be determined by eye with very good consistency. The stand-off ratio can be changed slightly by repeating pulsing to change slightly in the area of con-tact of small .area base one contact, if desired.

Referring now to FIGURE 3, there is shown a graph of the emitter characteristics of a typical unijunct-ion device made in accordance with the present invention. In the device the materials mentioned in connection with the device of FIGURE 1 were used and had the various parameters set forth in Table I. The graph shows the variation of voltage applied between the emitter and base one as a function of the current flowing between the emitter and base one with the value of voltage between base one and base two held constant as indicated, namely twenty volts. On the graph the various points described above are particularly designated. The peak point voltage and current refers to the voltage and current at point 15 of conduction of the emitter of the unijunc-tion transistor. The valley point current and voltage is the voltage existing at the point '16 at which the device becomes completely conductive. The saturation voltage and current refers to the current and voltage beyond the valley point and in this device is usually the diode voltage of the emitter and base one contact functioning as a diode. The speed of triggering refers to the time required for the voltage to drop from the peak point voltage to a point in the saturation region. The turn-off time is the time it requires the device to fully recover its quiescent condition after current has been flowing through it and emitter voltage is removed. The turn-on time is a function mainly of the spacing between the emitter 12 and the base one 11. For a given stand-off ratio, the spacing can be decreased considerably over conventional bar-type devices to decrease turn-on time without appreciably affecting other parameters of the device.

Turn-off time is a function of the lifetime property of the semiconductor Wafer. The lower the lifetime of the material, the faster the device will recover. However, if such a technique as gold doping of silicon is used to cut down the lifetime of the semiconductor Wafer, the saturation voltage would be increased. However, in view of the fact that the saturation voltage of the device of the present invention has been considerably reduced, an increase in the latter parameter over the reduced value still would provide a device having a saturation voltage considerably lower than in conventional devices of bar-type geometry. In the present device the turn-off time is of the order of a few microseconds.

While the device shown has been fabricated by means of alloy techniques, of course, it will be recognized that other techniques such as diffusion could as well be used to fabricate the geometrical configurations of the device. Of course, while the injecting contact has been shown as a dot, other geometries may as well be used. For example, this contact may be in the form of .a ring surrounding the small area contact.

The present invention is not limited to the particular embodiment described. For example, the emitter contact need not be located on the same surface. It may be located elsewhere in the body, however, the proportions specified for the small and large area ohmic contacts and the dimensions of the wafer must be met in order for spreading resistance to exist in the device, and the emitter must be located in the wafer to take advantage of the spreading resistance, as pointed out above. In FIGURE 4 wherein parts corresponding to parts of the device of FIGURES 1 and 2 are referenced by the same numerals, the emitter contact 12 is shown located on a side of the wafer. With the emitter located in the vicinity of the small area contact, the desired stand off ratio is obtained with a smaller separation of the base one contact and emitter than in conventional devices.

In FIGURES'S and 6, which shows another embodiment of the present invention, the body of semiconductor material 20 is a wafer which has an etched bore 21 in one face to obtain a small separation between one side and the other side of the semiconductor wafer. A small area contact 22 is made in the base of the bore 21. A rectifying contact 23 is made to the opposite surface in opposed relationship to the small area contact to provide the closeness of spacing between base one and the emitter for fast switch-on time. The base two contact 24 can conveniently be made to any part of the wafer remote from base one and is shown made to the opposite side, although it could as well be made to the end of wafer 20. Of course, the wafer is dimensioned to assure the existence of spreading resistance eifects in the vicinity of the small area contact. The device of FIGURES 5 and 6 would have very low switching time in view of the closeness of the emitter and base one and also would provide very low leakage currents between the various electrodes in view of the long surface paths between the various electrodes thereof.

Further improvements in the electrical characteristics of the device shown in FIGURE 6 can be made by means of a layer of lower resitivity material on the upper surface of the main body of semiconductor material 20 with a width somewhat less than the spacing between base one 22 and the emitter 23. This layer is of the same conductivity type as the main body of semiconductor material and may be formed by standard prosesses such as diffusion or epitaxial growth. With such a structure, higher stand-off ratios are obtained.

While specific embodiments of the invention have been described and shown, it .will, of course, be understood that various modifications may be devised by those skilled in the art which will embody the principles of the invention and be found in the true spirit and scope thereof.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a unijunction transistor including a body of semiconductor material of one conductivity type having a pair of base electrodes spaced on said body and making nonrectifying contact therewith; the improvement which comprises one of said base electrodes being small in area of contact in relation to the dimensions of said body and in relation to the area of contact of said other base electrode; and an additional electrode making rectifying contact with said body and being spaced closer to said one base electrode than to said other base electrode; that portion of said body between said one base electrode and said additional electrode having, to current flow between said base electrodes, an electrical resistance including the spreading resistance of said one base electrode which, in the absence of injection of minority carriers into said body from said additional electrode, is greater than the electrical resistance of that portion of said body between said additional electrode and said other base electrode.

2. In a unijunction transistor including a planar pellet of semiconductor material of one conductivity type having a pair of opposed sides; a small area ohmic contact to one of said sides; a large area ohmic contact to the other of said sides; and a zone of opposite conductivity type in said one side of said pellet and forming a PN junction therewith; the majority of resistance to current flow between said ohmic contacts, including the spreading resistance of said small area ohmic contact, lying in the portion of said pellet between the small area ohmic contact and said junction in the absence of injection of minority carriers into said pellet from said junction.

3. In a unijunction transistor including a planar pellet of semiconductor material of one conductivity type having a pair of opposed sides; a wire of small area cross section of said one conductivity inducing type fused to one surface of said pellet and forming a small area ohmic contact therewith; a large area ohmic contact to the other surface of said pellet; and a wire of opposite conductivity inducing type fused to a surface of said pellet between said sides and forming a PN junction therewith; the majority of resistance to current flow between said ohmic contacts, including the spreading resistance of said small area ohmic contact, lying in the portion of said pellet between the small area ohmic contact and said junction in the absence of injection of minority carriers into said pellet from said junction.

4. In a negative resistance exhibiting semiconductor device of the type having a body of semiconductive material provided with a pair of spaced ohmic contacts between which is adapted to be established a potential gradient, and having a PN junction through which minority carriers can be injected into the body to modulate the resistance of that portion of the body between the PN junction and the one ohmic contact toward which the minority carriers flow, and wherein the distance between the junction and said one ohmic contact can he traveled by said minority carriers during their lifetime and injection of minority carriers into said body from said junction produces a change in potential between said junction and said one ohmic contact which varies in inverse relationship with current flow therebetween, the improvement which comprises said one ohmic contact being smaller than said other ohmic contact and being located closer to said junction than said other ohmic contact, that portion of said body between said one ohmic contact and said PN junction having to current flow between said ohmic contacts a resistance including the spreading resistance of said one ohmic contact which in the absence of injection of minority carriers into said body from said PN junction is greater than the resistance of that portion of said body between said PN junction and said other ohmic contact.

References Cited by the Examiner UNITED STATES PATENTS 2,769,926 11/1956 Lesk 307-88.5 2,947,925 8/1960 Maynard et a1. 317-235 2,975,342 3/1961 Rediker 317234 2,993,126 7/1961 Dorendorf 30788.5 3,025,438 3/1962 Wegener 317--234 3,081,421 3/1963 Roka 307-885 3,114,867 12/1963 Szekely 307-88.5

FOREIGN PATENTS 1,037,293 9/1953 France.

JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, Examiner. 

1. IN A UNIJUNCTION TRANSISTOR INCLUDING A BODY OF SEMICONDUCTOR MATERIAL OF ONE CONDUCTIVITY TYPE HAVING A PAIR OF BASE ELECTRODES SPACED ON SAID BODY AND MAKING NONRECTIFYING CONTACT THEREWITH; THE IMPROVEMENT WHICH COMPRISES ONE OF SAID BASE ELECTRODES BEING SMALL IN AREAS OF CONTACT IN RELATION TO THE DIMENSIONS OF SAID BODY AND IN RELATION TO THE AREA OF CONTACT OF SAID OTHER BASE ELECTRODE; AND AN ADDITIONAL ELECTRODE MAKING RECITFYING CONTACT WITH SAID BODY AND BEING SPACED CLOSER TO SAID ONE BASE ELECTRODE THAN TO SAID OTHER ELECTRODE; THAT PORTION OF SAID BODY BETWEEN SAID ONE BASE ELECTRODE AND SAID ADDITIONAL ELECTRODE HAVING, TO CURRENT FLOW BETWEEN SAID BASE ELECTRODES, AN ELECTRICAL RESISTANCE INCLUDING THE SPREADING RESISTANCE OF SAID ONE BASE ELECTRODE WHICH, IN THE ABSENCE OF INJECTION OF MINORITY CARRIES INTO SAID BODY FROM SAID ADDITIONAL ELECTRODE, IS GREATER THAN THE ELECTRICAL RESISTANCE OF THAT PORTION OF SAID BODY BETWEEN SAID ADDITIONAL ELECTRODE AND SAID OTHER BASE ELECTRODE. 